Such a method and such an apparatus are basically already known from the EP-06 321 742. In the case of the known apparatus, a stationary receiving vessel is provided, in the bottom region of which comminuting and mixing equipment, which can rotate about the axis of the vessel, and a casing opening as outlet for the granulate produced in the apparatus are disposed. From there, the plastic material is supplied to a two-stage screw conveyor and, if necessary, maintained at a particular temperature.
Such an apparatus neither ensures adequate mixing, in order to make possible the use of recycled material even for the manufacture of high-grade plastic products nor, because of the absence of the possibility of interim storage, does it permit the use of a plunger-type injection unit, which operates discontinuously.
From U.S. Pat. No. 2,382,655, a method and an apparatus for recycling plastic waste are known. In this case, the material is comminuted in an apparatus provided with knives and is subsequently supplied by a screw conveyor to an apparatus, in which it is processed further.
U.S. Pat. No. 4,591,467 already discloses a two-step mixing, heating and homogenizing of plastic material.
The German patent 960,854 discloses an injection molding machine with a heating injection cylinder, in which a constantly revolving screw is disposed, which conveys the pre-plasticized material into the pressure space of an extruder. This injection molding machine cannot be used in this way for recycled material, if for no other reason than that it cannot ensure adequate mixing of the components.
The German utility patent 1,847,990 also discloses an injection molding machine for thermoplastic materials with screw plastification. However, because of insufficient premixing of the recycled components, this machine also cannot be used for a recycling method.
Plastics have gained increasing importance in all areas of everyday life, and also in the industrial sector.
Meanwhile, the disposal of parts produced from plastics increasingly is creating problems, since these parts, unlike natural products or parts derived from natural products, are not returned to the cycle of nature by decaying.
The significance of plastics is to be explained using the automobile as example. In a headlong development, plastics have meanwhile attained 10% by weight. Consideration of how this modern material could be used again did not keep pace with the headlong pace described previously. While steel, iron and aluminum, which constitute about 75% of the vehicle mass, can be largely used again, plastics are a burden on waste disposal sites. Furthermore, the danger exists that the plastic portions, because of the relationships described above, will become afflicted with a negative image in the eyes of the general public and, with that, lose acceptance by the people.
There is finally the danger that these modern materials will possibly have to be replaced for political reasons, although the engineers are not readily prepared to do without these materials, which moreover can hardly be replaced in the case of many components.
Recycling plastics therefore is the dictate of the hour. The way to recycling plastics passes through the steps of dismantling and pre-sorting, so that the plastics are obtained largely by type. For example, a large automobile plant in Wackersdorf is planning a dismantling plant. This is being done in the knowledge that, without reusing the plastics, there will be a greater need to resort to the possibility of burning of the plastic waste, which is, however, undesirable for environmental-political reasons.
Within the framework of plastics recycling, the largest garbage sorting plant in Europe, which will be designed to handle approximately 100,000 tons of garbage annually and in which the garbage components are to be separated into glass, metal, plastics, etc., is to be set up in Berlin. The pre-sorted raw materials are then made useful once again as "secondary raw materials".
Particularly in the packaging industry are there large amounts of plastic waste, which consists mainly of plastic material, which has had only a single heat treatment. This plastic material is of high quality, since the high-performance processing machines in the packaging industry can be operated only with such material.
The desire to recycle plastics has actually existed already for a long time. Up to now, however, all the projects tackled have failed because of the high costs of the processing. The recycled material, processed by previously conventional methods, is almost as expensive as new material. This is understandably a bad starting situation for the absolutely essential recycling.
Usually, the material, which is to be processed again, is collected, sorted, milled, extruded in an intermediate step and granulated, as well as subsequently, during the processing, melted down again and once more converted into the desired shapes. The renewed melting process alone causes considerable costs, because of the high energy consumption.
Adequate pre-sorting initially is a sufficient prerequisite for recycling plastics. The intensity of the pre-sorting selected depends on the purpose, for which the recycled material is to be used later on. Moreover, adequate mixing of the pre-sorted material is required, since the material cannot be regarded as homogeneous in spite of having been pre-sorted; the material frequently is not even all of one type of plastic. In the crude material, which consists largely of polyethylene, larger portions of polypropylene (such as seals, plugs, bottle tops), polystyrene, polyester, etc. are contained.
Only good homogenization of the raw material creates the conditions for recycling the plastics later on. It should, after all, also be possible to produce high-quality finished parts from the "secondary raw material", which possibly require admixtures of talc, glass fibers, general reinforcing agents, stabilizers and pigments or dyes.